This invention relates in general to methods for coating materials and in particular to a new and useful method.
It is known to produce such coatings by a reactive, ion supported evaporation in vacuum, and particularly a so-called low voltage arc discharge has proved suitable for the evaporation, if too strong a heating of the substrates is to be prevented. That is, because of the strong activation of the reactants in the low voltage arc, a satisfactory reaction is obtained even on cooled substrates, which is not easy with other reactive deposition process. One difficulty still arises, however, namely in the color of the coatings. The present invention is directed to a method of producing gold-color coatings on substrates, which can be reproduced with great accuracy in constant and uniform color hues. This uniformity is particularly important in the series manufacture of gold-color parts which are to be interchangeable within close tolerances. A more particular objective is to produce coatings in golden hues which are standard in the watch making industry, and to achieve a smaller reject rate and thus a better economy as compared to prior art methods. At the same time the novel method should assure a particularly careful treatment of the substrates during the coating operation, to be able to also treat substrates which, because of their sensitivity to heat, cannot be coated in conventional ways, for example in the chemical vapor deposition process (CVD); (this more particularly includes hard-chromium plating where the hardness is lost at temperatures above 400.degree. C. Alloys such as brass, and some plastics also are thermally sensitive substrates).
Deposition of gold-color layers of mere titanium nitride by reactive evaporation, with a simultaneous activation of the residual gas by means of an electric gas discharge has been proposed by applicant already in an earlier application, and the layers obtainable with this older method already represented a substantial advance in obtaining desired hues. The present invention is intended as a further step in this direction.